(1) Field of the Invention
The present invention relates to a process and apparatus for thawing a frozen food such as frozen meat or fish.
(2) Description of the Related Art
As the conventional thawing process, there can be mentioned (a) a process using hot air, (b) a process using microwaves, and (c) a heat radiation process. In the hot air process (a), if the temperature of hot air is too high, the surface portion of the frozen food is degenerated. Although it is considered that this disadvantage will be overcome if low-temperature air is used, a long time is required for the thawing in this case and the process is not applicable when rapid assortment or cooking is required. In the microwave process (b), the frequency that can be used at present is suitable for rotational vibration of the dipole moment of water, but this frequency is not optimal for ice and thus heat converges on the portion of water produced by thawing and the treatment effect becomes uneven. Namely, the central portion of the food remains frozen while the surface portion is boiled. In the heat radiation process (c), the thawing is effected by heat radiation from a surrounding heating plate, and far infrared radiation is especially effective. Nevertheless, this process has a problem in that the surface portion is readily deteriorated, the freshness is lost, and discoloration occurs, and a long time is required for completion of the thawing treatment.
Of the above-mentioned conventional thawing processes, the far infrared ray irradiation process is regarded as most advantageous, in that the efficiency of absorption of the far infrared ray energy is high and the energy is directly transferred to the interior of the food from the surface by heat radiation. But this process involves the problem of a loss of freshness and the long time required for completion of the thawing treatment.
The microwave process is effective for elevating the temperature of a frozen food, for example, from -30.degree. C. to about -3.degree. C. if a frequency suitable for ice can be adopted. But if the surface of the food becomes wet, a conduction electric current flows to weaken the electric field in the interior, and the surface portion is boiled because of a temperature difference between the surface portion and the interior. Therefore, it is difficult to uniformly and efficiently elevate the temperature from the frozen state at about -3.degree. C. to the thawed state above 0.degree. C. As a means for overcoming this disadvantage, a process is adopted in which a frozen food is irradiated with microwaves while cooling the surface portion of the food (see, for example, Japanese Unexamined Patent Publication No. 54-36660, Japanese Unexamined Patent Publication No. 60-105478 and Japanese Unexamined Patent Publication No. 60-259171). But since the specific heat of the cooling air is low and the cooling capacity of the cooling air is readily lost by elevation of the temperature by heat received from the frozen material, it is necessary in this process to at a level lower than -10.degree. C., and heat of the entire frozen food is taken by this cooling air, resulting in a reduction of the thawing efficiency. Therefore, the size of the apparatus must be increased, and accordingly, the cost is increased, and thus this process is practically worked only as a large-scale thawing machine for industrial purposes.